Hint: Threshold membrane is basically the minimum stimulus needed to achieve an action potential. At threshold voltage, voltage-gated channels become activated.
Action potential occurs when the membrane potential of a cell rises and falls. Talking of stimulus, it is a response or change in the body, object or a system depending on the external surroundings. There are certain types of stimulus.
a) Conditioned stimulus: This stimulus acquires the ability to evoke a given response by repeatedly being linked with another stimulus which naturally evokes the response.
b) Depolarizing stimulus: This stimulus lowers the rising potential making the fiber less negative.
Unconditioned stimulus: This stimulus naturally evokes a natural response.
c) Eliciting stimulus: Any condition that elicits a response either conditioned or unconditioned.
d) Threshold stimulus: Any stimulus strong enough to elicit a response.The response could be external or internal
-Internal: internal : temperature, metabolism
-External: touch, sensitivity to light, taste.
A twitch in the muscle occurs as a response to the stimulus in the nervous system. This stimulus affects all the nerve fibres. If there is not enough threshold, the muscle will remain relaxed. If the threshold is enough, it will contract.
The stimuli other than nerve threshold stimulus could be shock as in electrical stimulus, drugs and compounds as in chemical stimulus and pressure as in mechanical stimulus.
Note: Stimulation basically creates a response or activity. It can be physical or chemical. The strength of stimulus tells the reaction of that stimulus. The number of muscle fibres contracting determines how much force the whole muscle produces.